Mineral wool products are widely used for the thermal and acoustic insulation of different parts of buildings, transportations, or other appliances, as well as for fire protection. Mineral wool materials are mainly randomly interlaced masses of mineral fibers with varying lengths and usually bound by a resin-based binder. Three types of mineral materials are most commonly employed, glass, stone or slag. Mineral wool is formed by an intricate network of fibers which might be bonded in their cross-over points by different means, e.g. by using a cured binder. Processes for the production of mineral wool products are well known in the art, and usually comprise the steps of melting the mineral material to an adequate temperature, fiberizing the molten mixture into fine fibers, application (e.g. spraying) of a binder composition to the individual fibers, collection of the fibers and formation of a primary fleece on a foraminous conveyor, densifying the fleece, and curing the binder at elevated temperatures. The cured mat is then cut to the desired size with transverse and edge trimmers and optionally rolled up, before it is packaged for transport.
The type and amount of binder used to bond the fibers in the mineral wool plays an important role in the final properties of the produced mineral wool. A variety of binder systems have been described in the art, including binders based on melamine urea formaldehyde resins, furane-based resins, and others, but phenol-formaldehyde resin based binders have been preferred for very long time, due to their good performance and attractive low cost.
Environmental and toxicological concerns related to the formaldehyde released from the products manufactured with phenol-formaldehyde resin based binders have however led to a movement in the mineral wool industry to reduce or completely eliminate the use of such resins, or to reduce the amount of excess formaldehyde they contain.
In this regard, different formaldehyde-free resins have been described in the art, which are claimed to reduce formaldehyde emissions to zero. Some illustrative examples of these alternative resins are a) resins based on the esterification reaction between carboxylated polymers (e.g. acrylic copolymers) and low molecular weight polyfunctional alcohols, as those resins described in EP0583086B1 and U.S. Pat. No. 6,071,994A; b) resins using renewable carbohydrate materials reacted with a low molecular weight polyacid as a crosslinker, e.g. those described in US20100282996A1; and c) resins based on the Maillard reaction between a reducing sugar, and acid precursor and a source of nitrogen, as for example those described in WO200919232A.
Starch and its derivatives, due to their large availability at low cost, have also been explored as the renewable component for formaldehyde-free resin based binder compositions. Curable starch-based resins have exploited the fact that starch has a large number or hydroxyl-groups distributed along its polysaccharide chains which could potentially take part in curing reactions. For instance, these hydroxyl-groups have been targeted for crosslinking reactions of starch with carboxylic groups of organic polyacids. The suitable organic polyacids can be polymeric polycarboxylic acids (especially polyacrylic acid and its derivatives) and/or low molecular weight polycarboxylic acids (e.g. citric acid) as described in EP0911361B1 and US20090275699A1 and WO2008053332A1.
Native starch is a specific type of starch particularly interesting from the industrial perspective, due to its wide availability, reduced price, easy handling and long storage stability. Native starch does not require to be modified chemically, or by other means, in forehand to its use. A specific property that makes the use of native starch attractive for curable formaldehyde-free resins, which are to be used as base for binders for mineral wool products, is its higher resistance to degradation in humid conditions compared to the modified starches, starch derivatives (e.g. dextrins) and other saccharides, what at the end translates in a better resistance to weathering influences and in a better ageing behavior of the bonded mineral wool products. This is due in part to the ultra-high molecular weight of native starch and its granular structure. However, native starch presents several drawbacks which have made difficult its use in this type of applications, like the low solubility in cold water or the instability of its dispersions, as well as the high viscosity of the solutions obtained after solubilization (e.g. after gelatinization, hydrolysis, etc.).
It has recently been described by the applicant (application number PCT/EP2013/063660) that a curable binder composition comprising water insoluble native starch and a polycarboxylic polymer component can be prepared in form of dispersions in water. The so prepared binder compositions can be sprayed over the mineral fibers and cured for the manufacture of mineral wool products. However, this binder system has some limitations.
A particular downside of these resins and binders is related to the viscosity of the dispersions obtained. In general, when using resins in binders for mineral wool, it is desired that their viscosity is as low as possible. The low viscosity, in addition to facilitate handling of the resin, e.g. by pumping, mixing, etc., it also has an effect in the properties of the mineral wool products obtained after curing. Thus, for instance, low viscosity enables better penetration of the sprayed binder into the forming felt of mineral fibers, and it assures the required homogeneity of the binder distribution onto the individual fibers and through the thickness of the mineral wool mat.
Even more importantly, a lower viscosity of the resin improves the distribution of the binder preferentially into the cross-over points between the fibers forming the mineral wool mat, what translates in better recovery of the thickness of the uncured primary fleece on the foraminous conveyor, and in better mechanical properties of the mineral wool product obtained after curing. When the individual fibers are coated with binder, e.g. by the spraying technique, these fibers are just being formed from a mineral melt by a fiberizer, and they are still warm. The remaining heat of the fibers produces a rapid evaporation of the water contained in the binder mixture, increasing its solid content, and thereby drastically increasing the viscosity of the binder composition on the fibers. Thus, in this situation, the higher the viscosity contribution of the resin, the lower the mobility of the binder towards the cross-over points between the fibers of the mat.
Because of the reasons explained above, the phenol-formaldehyde resins broadly and successfully used in binders for the manufacture of mineral wool products favorably have a viscosity well below 50 mPa·s, usually below 20 mPa·s, when measured at a solid content of 45-55 wt.-%.
Regarding the resins in form of a dispersion described in the patent application number PCT/EP2013/063660, the viscosity of the dispersion is increased when the proportion of polycarboxylic polymer, e.g. acrylic resin, is increased, due to the fact that these polymers usually are water soluble high molecular weight compounds. On the other hand, when the relative content of polycarboxylic polymer is reduced, particularly below 45 wt.-%, e.g. to obtain a low viscosity, the mechanical bonding properties of the cured binder are detrimentally affected. As a consequence, in this resin system, a compromise has to be reached between the compositions giving lower viscosity and the compositions producing enhanced mechanical bonding properties.
From the above it is evident that there is still a need for improved curable formaldehyde-free binder compositions for the manufacture of mineral wool products, particularly binders which are based on resins in form of dispersions comprising water insoluble native starch, which overcomes the limitations and downsides of this type of resins previously known in the art, and which optionally have a higher environmental sustainability and/or are more economical.
The inventors surprisingly found that very useful resins and binders in form of dispersions comprising water insoluble native starch can be obtained by the present invention, which show reduced viscosity, and in which the mechanical bonding properties obtained after curing are not detrimentally affected and even are significantly improved.